Mode of action and toxicological effects of the sesquiterpenoid, nootkatone, in insects

Repellent activity of essential oils to the Lone Star tick, Amblyomma americanum

BACKGROUND

The Lone Star tick, Amblyomma americanum is important to human health because of a variety of pathogenic organisms transmitted to humans during feeding events, which underscores the need to identify novel approaches to prevent tick bites. Thus, the goal of this study was to test natural and synthetic molecules for repellent activity against ticks in spatial, contact and human fingertip bioassays.

METHODS

The efficacy of essential oils and naturally derived compounds as repellents to Am. americanum nymphs was compared in three different bioassays: contact, spatial and fingertip repellent bioassays.

RESULTS

Concentration response curves after contact exposure to 1R-trans-chrysanthemic acid (TCA) indicated a 5.6 μg/cm2 concentration required to repel 50% of ticks (RC50), which was five- and sevenfold more active than DEET and nootkatone, respectively. For contact repellency, the rank order of repellency at 50 μg/cm2 for natural oils was clove > geranium > oregano > cedarwood > thyme > amyris > patchouli > citronella > juniper berry > peppermint > cassia. For spatial bioassays, TCA was approximately twofold more active than DEET and nootkatone at 50 μg/cm2 but was not significantly different at 10 μg/cm2. In spatial assays, thyme and cassia were the most active compounds tested with 100% and 80% ticks repelled within 15 min of exposure respectively and was approximately twofold more effective than DEET at the same concentration. To translate these non-host assays to efficacy when used on the human host, we quantified repellency using a finger-climbing assay. TCA, nootkatone and DEET were equally effective in the fingertip assay, and patchouli oil was the only natural oil that significantly repelled ticks.

CONCLUSIONS

The differences in repellent potency based on the assay type suggests that the ability to discover active tick repellents suitable for development may be more complicated than with other arthropod species; furthermore, the field delivery mechanism must be considered early in development to ensure translation to field efficacy. TCA, which is naturally derived, is a promising candidate for a tick repellent that has comparable repellency to commercialized tick repellents.

Ixodes scapularis Is the Most Susceptible of the Three Canonical Human-Biting Tick Species of North America to Repellent and Acaricidal Effects of the Natural Sesquiterpene, (+)-Nootkatone

Ticks are vectors of many human and animal zoonotic disease-causing agents causing significant global health and economic strain. Repellents and acaricides are integral to the human capacity for personal protection from tick bites. Nootkatone, a naturally occurring sesquiterpene found in the Alaskan cedar tree, grapefruit, and other sources, has been documented to be a potent acaricide. Research has also noted repellent effects against some tick species. In this study, our aim was to investigate the effect of synthetic, high-purity (+)-nootkatone on adult Ixodes scapularis, Dermacentor variabilis, and Amblyomma americanum ticks in an in vitro, vertical filter paper bioassay. (+)-nootkatone showed compelling tick repellency, but median effective concentrations (EC50) significantly differed among species. Ixodes scapularis were repelled at very low concentrations (EC50 = 0.87 ± 0.05 µg/cm2). Higher concentrations were required to repel D. variabilis (EC50 = 252 ± 12 µg/cm2) and A. americanum (EC50 = 2313 ± 179 µg/cm2). Significant post-exposure mortality, assessed 24 h after repellency trials, was also observed in I. scapularis but was absent entirely in D. variabilis and A. americanum. These tests demonstrate that nootkatone has a promising dual-action personal protection capacity against adult I. scapularis ticks, warranting further investigation in more natural environments and in the presence of host cues.

Cuticular property affects the insecticidal synergy of major constituents in thyme oil against houseflies, Musca domestica

Plant essential oils are intricate blends comprising predominantly of monoterpenes and some sesquiterpenes. These oils display diverse bioactivities against targeted organisms, often arising from complex interactions among their constituents, which may demonstrate synergistic or antagonistic effects. Despite their wide use as botanical insecticides, the mechanisms behind these interactions and their effects on bioactivity are poorly understood. This study investigated the synergistic interaction of thymol and p-cymene, two major constituents of Thymus vulgaris essential oil, on the larvae and adults of the housefly, Musca domestica. The results showed that p-cymene synergized the insecticidal activity of thymol in adult houseflies, but not in larvae. GC-MS analyses and bioassays indicated the increased cuticular penetration of thymol by p-cymene was the mechanism of synergy, which was observed only in the adults. Two potential routes were proposed: the expansion of the wetting area, or the disruption of cuticular integrity through dissolving the wax layer. The sequential application and large-volume treatment bioassay results suggested that the former was the more likely mechanism. Also, the hydrophobicity of the cuticle seemed critical for this stage-specific synergy. Wax-devoid adults failed to show synergistic toxicity, whereas artificially wax-coated larvae gained a synergistic effect. Overall, the findings provide insights into the synergistic mechanism of insecticidal activity of plant essential oils and suggest potential applications in developing effective strategies using penetration-enhancing synergists.

Functional interactions between potassium-chloride cotransporter (KCC) and inward rectifier potassium (Kir) channels in the insect central nervous system

The K⁺/Cl^- cotransporter (KCC) is the primary mechanism by which mature neurons maintain low intracellular chloride (Cl^-) concentration and has been shown to be functionally coupled to the GABA-gated chloride channels (GGCC) in Drosophila central neurons. Further, pharmacological inhibition of KCC has been shown to lead to acute toxicity of mosquitoes that highlights the toxicological relevance of insect KCC. Yet, gaps in knowledge remain regarding physiological drivers of KCC function and interactions of ion flux mechanisms upstream of GGCC in insects. Considering this, we employed electrophysiological and fluorescent microscopy techniques to further characterize KCC in the insect nervous system. Fluorescent microscopy indicated insect KCC2 is expressed in rdl neurons, which is the neuron type responsible for GABA-mediated neurotransmission, and are coexpressed with inward rectifier potassium (Kir) 2 channels. Coexpression of Kir2 and KCC2 suggested the possibility of functional coupling between these two K⁺ flux pathways. Indeed, extracellular recordings of Drosophila CNS showed pre-block of Kir channels prior to block of KCC led to a significant (P < 0.001) increase in CNS firing rates over baseline that when taken together, supports functional coupling of Kir to KCC function. Additionally, we documented a synergistic increase to toxicity of VU0463271, an established KCC inhibitor, above the expected additive toxicity after co-treatment with the Kir inhibitor, VU041. These data expand current knowledge regarding the physiological roles of KCC and Kir channels in the insect nervous system by defining additional pathways that facilitate inhibitory neurotransmission through GGCC.

Production, Function, and Applications of the Sesquiterpenes Valencene and Nootkatone: a Comprehensive Review

Valencene and nootkatone, two sesquiterpenes, extracted from natural sources, have great market potential with diverse applications. This paper aims to comprehensively review the recent advances in valencene and nootkatone, including source, production, physicochemical and biological properties, safety and pharmacokinetics evaluation, potential uses, and their industrial applications as well as future research directions. Microbial biosynthesis offers a promising alternative approach for sustainable production of valencene and nootkatone. Both compounds exert various beneficial activities, including antimicrobial, insecticidal, antioxidant, anti-inflammatory, anticancer, cardioprotective, neuroprotective, hepatoprotective, and nephroprotective and other activities. However, most of the studies are performed in animals and in vitro, making it difficult to give a conclusive description about their health benefits and extend their application. Hence, more attention should be paid to in vivo and long-term clinical studies in the future. Moreover, valencene and nootkatone are considered safe for consumption and show great promise in the applications of food, cosmetic, pharmaceutical, chemical, and agricultural industries.